The Chinese have built this swing on top of a 700 foot tall viewing platform in China. Swingers are propelled out over the edge of the platform for a terrific (or terrifying) view of Harbin city in the Heilongjiang province. Link

While this may be the swing with the longest potential fall, the world's highest swing highlighted here in March still seems much scarier (particularly with its stomach turning 33 story freefall).